They have purchased rights to use the novel as a base for the movie, and the visual design of howl is theirs. So the particular movie and its visuals are copyrighted to Ghibli, altho the novel was where the plot of the movie originated from.

If you want more indepth discussions about the issue, I think Studio Ghibli's lawyers would be the right people to contact. What matters to me is that rights have been purchased, the studio has right to use the names and plot, and owns the copyrights to their version and design, which is the version I made fanart of. I didn't make fanart of the novel, I've never read it. I made fanart of a derivative work, not the original.